1. Field of the Invention
The present invention concerns a magnetic resonance apparatus of the type having a cylindrical body coil formed by a number of resonator segments distributed around the circumference and a control device for separately activating of the individual resonator segments (the resonator segments being electromagnetically decoupled from one another) so as to generate a radio-frequency field inside the coil for magnetic resonance (MR) spin excitation in an examination volume.
2. Description of the Prior Art
Magnetic resonance tomography is one of the imaging methods in medical diagnostics that uses the interaction of an external field (here a magnetic field) with the human body for imaging. Known magnetic resonance apparatuses have a (normally) cylindrical body coil that is formed by a number of resonator segments distributed around the circumference. Each resonator segment normally is formed of at least one capacitor as well as a conductor element that proceeds parallel to the longitudinal axis of the surrounding basic field magnet. A conductor segment is appropriately fashioned as a band conductor; the multiple conductor segments being distributed around the circumference of the body coil. Coils are known in which the resonator segments are electromagnetically decoupled from one another, such that each resonator segment can be separately activated by a control device.
A homogeneous, radio-frequency excitation magnetic field that serves for tilting the nuclear spins in the patient to be examined is now generated inside the body coil by the body coil or the resonator segments thereof. The body coil can also be oval in cross-section or deformed in another manner as an alternative to the cylindrical shape. The design and the functioning of such a magnetic resonance system is known and does not have to be explained in detail herein.
For implementing a series of magnetic resonance examinations, standard localized saturation pulses are used that are generated via the body coil so that selected body regions do not appear in the magnetic resonance image. This is necessary, for example, for suppression of motion artifacts, for example in a spinal column acquisition in which the abdominal wall moving due to respiration (which would lead to artifacts in the image region of the spinal column) is suppressed with localized saturation pulses. A further circumstance that can lead to image artifacts is fold-over artifacts in the phase encoding direction when, for time reasons, the entire excited imaging region is not to be acquired with a sufficiently dense sampling of k-space.
Such a saturation pulse is typically integrated into the excitation sequence and is activated prior to the actual excitation pulse that serves for the generation of the excitation field. Gradient pulses are activated in parallel with the saturation pulse; the spatial selectivity is achieved in this manner. This saturation pulse serves to specifically destroy or to dephase the magnetization in a specific volume. Due to the saturation pulse, the magnetization in the excited examination volume region exists in the transversal plane and there is cancelled therein by one or more subsequently-activated gradient pulses (“spoiler pulses”). The actual excitation pulse that serves for the actual image acquisition subsequently ensues. The entire examination volume or a volume slice is thereby excited by the body coil by an RF field being generated in the entire interior of the body coil. A spin excitation is possible, however, only in the non-saturated region of the examination volume, thus in the region that is not saturated by the saturation pulse; no spin excitation can occur in that region. This means that, in the prior art, a double sequence always ensues for acquisition of an image, namely a first excitation sequence to provide the saturation pulse in order to generate a saturation volume that can no longer be excited for imaging, and subsequently the actual excitation pulse in the entire volume or a volume slice, thus also in the region of the saturated volume, but no image information can be acquired due to the dephasing.
This excitation sequence is time-consuming because two separate pulses must be provided. Moreover, a relatively large amount of energy is radiated into the patient, which increases the specific absorption rate SAR, which is associated with limitations with regard to the number of slices, the repetition time or the measurement time. The problem also exists that the dephased magnetization may possibly be re-phased with the subsequent excitation pulse, such that in spite of the saturation pulse it can contribute to the image information as an image artifact.
A magnetic resonance apparatus is known from DE 101 24 465 A1 with a body coil having a number of resonator segments distributed around the circumference and electromagnetically decoupled from one another. Separate transmission channels are respectively associated with the resonator segments such that the phase and the amplitude of the RF feed can be individually predetermined for each resonator segment, which enables a nearly complete control of the radio-frequency field distribution in the examination volume.
Another method for operation of a magnetic resonance apparatus is known from DE 10 2004 002 009 A1, in which the B1 field (RF field) distribution is measured in at least one sub-region of an examination volume of a radio-frequency antenna of the magnetic resonance apparatus, and the RF pulses emitted by the radio-frequency antenna for homogenization in a specific volume are then optimized on the basis of the determined B1 field distribution. For this purpose, an effective volume within the examination volume is determined beforehand for each applied RF pulse and individual adjustment on the basis of the determined B1 field distribution for the appertaining RF pulse in that volume is undertaken, such that the B1 field is homogenized within the effective volume of the RF pulse. A best-possible mode of operation of each applied RF pulse is thereby achievable. The radio-frequency antenna itself has a number of individual antenna elements, and in the framework of the described pulse mode optimization the B1 field is respectively measured separately for all antenna elements in order to determine the effect of the individual antenna elements within the examination volume.